Polyphenylene ethers (also known as polyphenylene oxides) are a class of polymers widely used in industry, especially as engineering plastics in applications requiring toughness and heat resistance. In recent years there has developed an increasing interest in employing polyphenylene ethers in food packaging applications.
In many of these food packaging applications, it is essential that the polyphenylene ether be free from materials which are volatile, have undesirable odors or would otherwise harm the food. Various materials of this kind may be present in polyphenylene ether resins. They include dialkylamines such as di-n-butylamine, which are components of the catalyst used in the preparation of polyphenylene ethers as described hereinafter. Also present may be by-products formed in the synthesis of the substituted phenols from which the polyphenylene ethers are prepared. In the case of poly(2,6-dimethyl-1,4-phenylene ethers), these frequently include 2,4,6-trimethylanisole (2,4,6-TMA); 7-methyldihydrobenzofuran (7-MDBF); 2,3-dihydrobenzofuran; 2,6-dimethcyclohexanone and 2-ethylhex-2-enal. Removal of 2,4,6-trimethylanisole is particularly crucial because of its pronounced odor.
Conventionally, polyphenylene ether is manufactured by using a recycled liquid aromatic hydrocarbon solvent, which has a high concentration of odoriferous compounds, due to the continuous build up of impurities in the solvent, as compared to odoriferous concentrations in the polyphenylene ether monomer. A high concentration of odoriferous materials in the polyphenylene ether resin results, causing undesirable taste and odor and thus inhibiting the use of the resin in food contact applications, as little as 10 parts per million by weight can be detected by certain individuals with highly developed olfactory systems. Five parts per million and less are normally not detectable.
The issue of unpleasant odor associated with polyphenylene ether resin has been a longstanding problem with many plastics processors. Recently, extensive work has been carried out in order to reduce odor in polyphenylene ether resins. The primary area of focus of this work has been the removal of residual amines either by devolatilization during extrusion or post-extrusion extraction. See Kasahara et al., U.S. Pat. No. 4,369,278; Newmark, U.S. Pat. No. 3,633,880; Abolins and Hasson, U.S. patent application Ser. No. 210,733, filed Jun. 23, 1988, and Bopp, U.S. patent application Ser. No. 206,174, filed Jun. 13, 1988.
The present invention sets forth a method to produce a low odor polyphenylene ether resin in the initial production stage by reducing phenolic by-products of the monomer synthesis, such as 2,4,6-trimethylanisole and 7-methyldihydrobenzofuran, but not the reduction of residual amine components during post-production extrusion stages.